Where is the alliance party?

Alliance party is an event that is available for a day but needs to be stared by an alliance R4/5. It then runs for 20 minutes during which time you will see an icon in the lower right panel that you can tap to join. There is also a message in the alliance chat that you can tap on. You can check with your alliance leaders on the appropriate day what time they will run it if you want to make sure to join.